<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript" src="script/jquery-1.7.2.js"></script>
    <script>
        $(function () {
            $("#username").val()
        })
    </script>
</head>
<body>
    <div>div1</div>
    <div>div2</div>
    <div>div3 <span>div中的span</span></div>

    <input type="text" name="username" id="username" />

</body>
</html>